American Society of Safety Engineers
, American Engineering Standards Committee
and United States. Bureau of Labor Statistics.
Safety Code for the Construction, Care and Use of Ladders : Bulletin of the United States Bureau of Labor Statistics, No. 351 ,
Washington, D.C:
G.P.O.,
October 1923, https://fraser.stlouisfed.org/title/3995,
accessed on November 19, 2018.